Dispersal and Geochemistry of Surface Sediments in Halls Bay, North-Central Newfoundland: Application to Mineral Exploration

Abstract

Surficial palimpsest sediments in Halls Bay, north-central Newfoundland, are mixtures of gravel, sand, and mud deposited from a number of sources in varying quantities from late Wisconsinan to the present time. Shallow water gravel originated as till and glacio-fluvial outwash. Gravel in deep water probably is ice-rafted. Sand and mud, which occurs with shallow water gravel and in deeper water, is a combination of fluvial material and material winnowed out of till and outwash by shallow water waves and currents during early marine transgression. There also may be a contribution of fine-grained sediment from the adjacent shelf.Gravel (coarser than [Formula: see text]), very fine sand (3 to [Formula: see text]) and coarse silt (4 to [Formula: see text]) modal grain-size classes predominate in the sediments. The very fine sand mode occurs on the west side of the inlet and the coarse silt mode occurs on the east side regardless of water depth, indicating net or active easterly dispersal of fine-grained sediment. This dispersal path may result from the presence in Halls Bay of a counterclockwise gyre of the Labrador Current that has developed since early transgression, which suggests the sediment surface is adjusting to the Halls Bay modern hydraulic regime.Sandy and muddy sediments are composed of quartz, feldspar, amphibole, illite, chlorite, montmorillonite, organic matter, CaCO3, and FeS. Major, minor, and trace element concentrations vary with grain-size, owing to the different proportions of these components in different size fractions. Calculation of an average chemical composition of sediments is biased because of this grain-size effect. The grain-size effect on chemistry of a suite of sediments can be accounted for by ratioing element concentrations to clay content.Plots of the ratio trace metal concentration/clay content vs. clay content for six trace metals indicate anomalous Cu concentrations occur in surface sediments along the east side of Halls Bay in the direction of fine-grained sediment dispersal. The anomalous Cu is derived from onshore mineralization in Lushs Bight Group volcanic rocks, which Occur along the west side of the inlet.The results provide an example of the applicability of marine sedimentologic/sedimentary geochemical investigations to mineral exploration. Local geochemical anomalies in sediments can be detected by routine analysis of total metal content of bulk samples provided the grain-size effect on chemistry is accounted for. The anomalous metal can be traced to its onshore source by evaluating sediment dispersal paths from textural variations.